Separation processes such as dialysis, ultrafiltration, and reverse osmosis have been used in the separation of a wide variety of impurities for solutions. The development and improvement of membranes for such systems has permitted their use, for example, in the desalination of brackish and saline waters.
The eminent success of the membranes used in permselective applications has prompted consideration of their use in the separation of gases. Such membranes are generally prepared in a water-wet condition, and various techniques have been tested for removal of the water to dryness. Direct drying techniques, however carefully controlled, seem unsatisfactory. The replacement of the water with a series of polar and non-polar liquids has met with some success in the drying of cellulose acetate membranes. However, a continuing need exists for a drying process for cellulose acetate and other cellulose ester membranes which is more efficient than the multi-step techniques previously used.